The present invention is directed to integrated circuits. More particularly, the invention provides an apparatus and method for generating differential signals. Merely by way of example, the invention has been applied to an analog-to-digital converter. But it would be recognized that the invention has a much broader range of applicability.
The analog-to-digital converters have been widely used for converting an analog input signal to a digital output signal. For example, the analog-to-digital converters are often required for providing analog signals to digital circuits. The analog input signals are usually continuous in time and amplitude, and the digital output signals are often discrete in time and amplitude.
Some analog-to-digital converters each include a comparator to successively compare an analog signal to several reference signals. These converters are often slow because a series of comparisons usually has to be performed in order to convert the analog signal to a digital signal. Other analog-to-digital converters are flash converters. The flash converters often use many comparators to simultaneously compare an analog signal to several reference signals. These comparators can simultaneously provide bits for a digital signal.
The conventional analog-to-digital converters often use differential mechanism. The differential mechanism converts a single-ended input signal to differential input signals. For example, the differential conversion is performed by a charge and transfer circuit. The charge and transfer circuit usually performs well for low and medium frequencies, but for high sampling frequencies, such as one that is higher than 100 MHz, the charge and transfer circuit often cannot easily achieve conversion accuracy better than eight bits. In another example, the differential conversion is performed by a high-speed operational amplifier. The operational amplifier often consumes significant current and introduces extra noises. In yet another example, the differential conversion is accomplished by a transformer. But the transformer cannot be easily integrated onto a digital circuit chip in many applications.
From the above, it is seen that an improved technique for generating differential signals is desired.